In a typical cellular system, also referred to as a wireless communications network, wireless terminals communicate via a Radio Access Network (RAN) to one or more core networks. The wireless terminals may be mobile stations or user equipment units such as mobile telephones also known as “cellular” telephones, and laptops with wireless capability.
The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks is also called Evolved Node B (eNB), NodeB or B node and which in this document also is referred to as a base station. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. A base station communicates over the air interface operating on radio frequencies with the user equipment units within range of the base stations.
In some versions of the radio access network, several base stations are typically connected, e.g., by landlines or microwave, to a Radio Network Controller (RNC). The radio network controller, also sometimes termed a Base Station Controller (BSC), supervises and coordinates various activities of the plural base stations connected thereto. The radio network controllers are typically connected to one or more core networks.
The Universal Mobile Telecommunications System (UMTS) is a third generation mobile communication system, which evolved from the Global System for Mobile Communications (GSM), and is intended to provide improved mobile communication services based on Wideband Code Division Multiple Access (WCDMA) access technology. UMTS Terrestrial Radio Access Network (UTRAN) is essentially a radio access network using wideband code division multiple access for user equipment units. The Third Generation Partnership Project (3GPP) has undertaken to evolve further the UTRAN and GSM based radio access network technologies. In 3GPP this work regarding the 3G Long Term Evolution (LTE) system is ongoing.
A Channel Quality Indicator (CQI) report describes the channel quality experienced in downlink by a user equipment, such as a user terminal. The downlink is the transmission path from the base station to the user equipment. The following CQI reporting description is for Long Term Evolution (LTE), but similar reporting procedure applies for Worldwide Interoperability for Microwave Access (WiMAX) as well.
A user equipment may be configured to send CQI reports to a base station either periodical, on Physical Uplink Control Channel (PUCCH), or scheduled, on Physical Uplink Shared Channel (PUSCH). The CQI for the last, i.e., most recent, reporting period may indicate forthcoming channel quality to a various degree of accuracy, but e.g. user position and user speed, altering the fast fading, as well as interference might change, causing the estimated CQI to be different from the correct one. This makes it clear that the estimated CQI has better short-term than long term accuracy, and that a long delay between measurement and CQI usage may reduce the accuracy when scheduling and selecting transmission format. A typical delay between measurement and application of CQI may be 10-15 ms. For example, the reporting delay may be 6 ms and the reporting interval may be in the order of 5 to 40 ms.
The user equipment bases the CQI reports on measurements of the gain to interference ratios on downlink reference signals. CQI reports are transmitted on uplink control channels to the scheduler situated in eNB. CQI reporting may be included in the Channel State Information (CSI) report. Depending on CSI report configuration, a CSI report may comprise a Pre-coding Matrix Indicator (PMI), a Rank Indicator (RI), one wideband CQI and sub band CQI offsets, in relation to the wideband CQI. A higher sub band CQI hence indicates a better sub band. There is also a user equipment selected best sub band CQI reporting where the user equipment only reports sub band CQI for best sub bands.
A scheduling algorithm, e.g. maximum Signal to Interference Ratio (Max-SIR), Proportional Fair (PF), etc., may utilize wideband or frequency selective scheduling (FSS) policies, hence use wideband or sub band CQI estimates.
Sub band CQI may also be used for more accurate transport format selection when scheduled on a fraction of the total available band. This can be the case in combination with FSS, but also for simpler scheduling algorithms such as round robin when there is a smaller amount of data to send which not requires the full bandwidth. This can be the case for example for voice over IP service which requires only one or two resource blocks per speech frame.
With frequency selective scheduling, it is possible to benefit from channel fading; since sub bands fade individually, the scheduler may select which sub band to use given their quality measure. But to make sure that the radio resource is efficiently utilized, it is important that proper sub bands are scheduled.
As a user equipment moves, the radio channel will be affected by the altering fast fading. For a given frequency, i.e. sub band, the channel will fade in the time domain and the fading speed is primarily dependent on the user equipment speed. If a specific point in time is considered, more or less similar occurrence may be seen, but in the frequency domain instead.
Due to the delays involved in the CQI reporting procedure, the sub band CQI to be used by e.g. the scheduler will be more or less outdated. The more outdated, the less representative will the CQI be, and hence, the corresponding performance using a non-representative CQI will likely be suboptimal. Having a too outdated CQI will be similar to have a random selection procedure; this is depicted in FIG. 1. FIG. 1 illustrates CQI distributions; an optimal selection procedure illustrated as a dashed line, where maximum CQI among all sub band per CQI reporting interval is selected without any delay, versus an approach where CQI is selected randomly among available sub bands, illustrated as a solid line.
US 2008/0057969 deals with optimization of CQI reporting time intervals. It describes a comparison of predicted CQI with real CQI to conclude if current reporting time interval is valid or not. Only the time evolution of the CQI values is considered.
US 2006/0270432 describes channel prediction using CQI and power control commands followed by scheduling. Again, only time evolution is considered.